A DYNAMIC-MODEL DESCRIBING LEAF TEMPERATURE AND TRANSPIRATION OF WHEAT PLANTS

A model of actual wheat canopy transpiration flux is presented as a function of the local transpiration flux at the top of the canopy, of leaf area index and of canopy nitrogen content. Three main assumptions are stated to derive this function. Firstly, with respect to a horizontal area element, the leaf area density and local transpiration flux are independent of their location within a canopy layer (homogeneity in horizontal direction). Secondly, the transpiration activity of plant leaves depends not only on driving environmental forces and available water but on leaf nitrogen content also. Thirdly, the profile of the nitrogen content of leaves within the canopy is divided into a ''top'' and into a ''bottom'' region. Both regions are vertically uniform within the canopy (rectangular profile). Based on these assumptions, the canopy transpiration flux of wheat plants grown in north Germany under farmer's usual fertilization (N4) and zero fertilization (NO) was investigated with the model. The influence of the normal N fertilization is twofold: transpiration increases because of the higher leaf area index, but also because of the higher nitrogen content of the leaves. A comparison of measured soil water contents under the two N treatment canopies with the modelled canopy evapotranspiration shows that the presented approach is appropriate to capture differences in water budgets due to differences in plant nutrition. The transpiration model is suitable for incorporation into complex models of plant canopies (water transport, growth).